A continuously variable transmission (CVT) is a power transmission providing infinite variability of speed ratios within a calibrated range. That is, while conventionally-geared transmissions typically use planetary gear sets and multiple rotating and/or braking clutches to establish a desired speed ratio, a CVT instead uses a variator assembly with a pair of variable-diameter pulleys to transition anywhere within the calibrated range of speed ratios. The pulleys are interconnected via an endless rotatable drive mechanism such as a chain or a drive belt. The drive mechanism resides in a variable diameter gap defined by conical sheave faces of the pulleys. One pulley is typically connected to an engine crankshaft and thus acts within the variator assembly as a driving/primary pulley. The other pulley is connected to an output shaft of the CVT to thereby act as a driven/secondary pulley. One or more planetary gear sets could be used on the input and/or output side of the variator assembly as needed to provide a desired power flow.
To vary the speed ratio in a CVT, a clamping pressure is typically applied to the primary and/or secondary pulley via one or more pulley actuators. The clamping pressure effectively squeezes opposing sheaves of the actuated pulley(s) together to change the width of the gap between the sheave faces. Variation in the gap size, which is also referred to as the pitch radius, causes the drive mechanism of the CVT to ride higher or lower within the gap depending on the direction of the gap size variation. The change in gap size changes the effective diameters of the variator pulleys and, as a result, ultimately determines the CVT speed ratio.
Certain emerging CVT designs are capable of achieving a fixed gear mode of operation in addition to a conventional friction drive mode. Fixed gear mode is achieved via the positive engagement of mating structure such as teeth or other suitable engagement features of the drive mechanism, the variator pulley shafts, and/or the conical faces of the pulley sheaves. Such fixed gear/positive engagement-type CVT designs increase the efficiency of the variator assembly relative to conventional CVT designs relying solely on friction between the drive mechanism and the pulley sheaves to transfer drive torque across the variator assembly.